Liquid crystal matrix displays, in particular, active matrix type liquid crystal displays provided with a switching element for each picture element have recently been researched and developed in many institutes for use in thin picture displays. A TFT of a MIS type is generally utilized in these displays.
FIG. 7 schematically shows an example of the structure of an active matrix type liquid crystal display which uses a TFT. Reference numeral 14 represents a TFT, and 15 a liquid crystal layer for one pixel which is clamped between a pixel electrode (not shown) connected to the drain electrode of the TFT 14 and a counter electrode 16 provided on a substrate which opposes the substrate provided with the TFT through the liquid crystal layer. The reference numeral 13 represents a gate wiring for connecting the gate electrode of each TFT 14 and supplying a scanning signal which turns each TFT on and off for each line to the gate electrode of the TFT 14. Reference numeral 11 denotes a source wiring for connecting the source electrode of each TFT 14 and supplying a picture signal to the source electrode of each TFT 14 which is selected by the gate wiring 13. The principle of display by an active matrix type liquid crystal display will now be briefly explained with reference to FIG. 7. For example, when a select signal is applied to a signal terminal Xi in the gate wiring 13, all TFTs 14-a which are connected to the terminal Xi are turned on at once, and a picture signal is supplied from the signal terminal Yi, Yi+1, . . . of the source wiring 11 to the pixel electrode which is connected to the drain electrode through the source electrode of each TFT 14-a. The voltage of the pixel electrode and the voltage of the counter electrode 16 determine the voltages applied to the respective liquid crystal layers 15, and the determined voltages change the light transmittances of the respective liquid crystal layers to produce the display. When the signal applied to the signal terminal Xi assumes a non-selected state and each TFT connected to the Xi is turned off, a select signal is applied to the subsequent signal terminal Xi+1, and the same operation as the above is effected. The voltage applied to each liquid crystal layer 15 is retained due to the capacity component of the liquid crystal layer 15 itself even after the TFT 14 is turned off until the same TFT is turned on.
A reverse stagger type a-Si TFT in which a gate electrode is disposed on the layer under a gate insulation layer and a semiconductor layer and a source electrode and a drain electrode are disposed on the layer above the gate insulation layer and the semiconductor layer, is widely utilized for the TFT 14. A reverse stagger type a-Si TFT having a structure in which a gate insulation layer, an amorphous silicon layer and a protective insulation layer are subsequently formed in that order has been specifically proposed from the point of view of reliability and reproducibility.
FIGS. 8 and 9 represent an a-Si TFT having the above-described structure together with the gate wiring, the source wiring, the pixel electrode, etc. In FIGS. 8 and 9, reference numeral 1 represents an insulating substrate, 2 a gate wiring, 3 a gate insulation layer, 4 an amorphous silicon layer, 5 a protective insulation layer, 6 a silicon layer containing impurities aimed at ohmic contact in the source and the drain and the blocking of a hole, 7 a metal layer for forming a source electrode and a drain electrode, and 8 and 9 a source electrode and a drain electrode composed of the impurity silicon layer 6 and the metal layer 7. Reference numeral 10 represents a transparent conductive layer, 11 and 12 a source wiring and a pixel electrode composed of the transparent conductive layer, and 13 a gate wiring.
In an active matrix type liquid crystal display, since the width of the source wiring 11 is about 10 to 20 micrometers, disconnection on the source wiring 11 sometimes occurs when there is dust, the end portion of the gate wiring 13 at which there is a difference in level is crossed,or the like. When disconnection occurs, the defective conduction leads to a line defect on the display screen which greatly deteriorates the display quality. For the transparent conductive layer 10, an ITO is generally used, but it makes it difficult to form a fine pattern at the time of etching, which is one of the causes of disconnection.
As described above, since the source wiring 11 is conventionally formed only from the transparent conductive layer 10, disconnection is apt to occur, and it is considered to be one of the causes of defective conduction.